This invention relates to watermarks that form part of the content on computer readable memory media. More particularly, this invention relates to memory media having watermarks and methods of creating and detecting such watermarks.
It is of great concern to the music, video, software and publishing industries to be able to prevent and track improper copying of content from computer readable memory media (e.g., digital video disks (DVDs), compact disks (CDs), read only memory (ROM), random access memory (RAM), various magnetic media and other memory media). This concern has been addressed, in part, by the use of “watermarks,” which are hidden messages arranged on a memory medium along with other data, such as video data, audio data, various software applications, etc.
For example, watermarks have been used on optical memory media, such as CD-ROMs and DVDs to prevent the undesired copying of music, movies and software. Notwithstanding such efforts, commercial pirates have used professional editing equipment to erase, obfuscate or otherwise bypass watermarks so that the desired content can be copied from the media on which the content is stored.
One way that pirates have bypassed existing watermark protection systems with optical media having digital data embedded thereon, such as CD-ROMs and DVDs, is through the use of editing and copying equipment that geometrically modifies the configuration (e.g., orientation, shape or size) of the digital content including the watermark in such a way as to render the watermark unreadable. Such geometric modification has been accomplished in many different ways including rotation, resizing, cropping, warping, mirroring, etc.
More specifically, with respect to digital video data embedded on a DVD or the like, the data is arranged in numerous consecutive video frames. Prior art watermarks that carried a “do not copy” message were arranged in one or more of the video frames to prevent unauthorized and undesired copying. However, consumer and professional playback, editing and copying equipment can be used to rotate and resize of the video data. The resizing factor and the rotation angle can vary over continuous ranges.
In the case where editing equipment is used to slightly rotate or resize the video data, the watermark in each frame is similarly rotated or resized. A data detector, such as a standard correlation detector or matched filter, has been used to detect the presence of any watermark pattern in each frame of video data. When the frames are rotated or resized, even a very small amount, the correlation detector often cannot properly detect the presence of a pattern within the watermark and thus the content, of the watermark message cannot be read. This occurs because prior art correlation detectors are particularly dependent upon specific registration of the digital data patterns of the watermark within each video frame. Thus, a slight modification of the resizing factor or rotation angle, can cause the failure of a correlation detector to detect the pattern and thus, the specific “do not copy” message of the watermark. The content on the DVD or other optical memory media could then be copied.
Attempts have been made to create robust watermarks that are resistant to efforts by pirates to overcome such watermarks and copy the accompanying data. Some of these attempts involve the use of log-polar coordinates and Fourier and Mellin transforms for image processing and registration. Data can be converted from normal Cartesian coordinates to log-polar coordinates through a known algorithm, which typically requires coordinate transformation from normal Cartesian coordinates (x, y) to polar coordinates (R, θ) and then to log-polar coordinates (L, θ) by taking the log of the radius after the coordinates have been converted from Cartesian to polar.
Unfortunately, these existing watermarking systems have various drawbacks. First, they require that the watermark be embedded in a particular transform domain for it to be resistant to geometric transformations. This limits the flexibility in the design of the watermark, and so these techniques cannot be incorporated into a previously designed watermarking system to improve it.
Second, these watermarking systems have been criticized for being robust to geometric transformations, but not to other attacks such as noise addition. The technical reason is that these systems obtain geometric robustness by embedding the watermark in the magnitude of a Fourier transform. This transform magnitude is invariant to spatial shifts in the input to the transform, but it is easy to modify and attack. On the other hand, it is well known in image processing that the phase of the Fourier transform of an image contains most of the information in the image. It is possible to completely change the magnitude of the transform, inverse transform the magnitude and phase, and still see much of the content of the original image.
Other approaches embed simple patterns, or complete watermarks, at known positions in images or video frames, and then detect these patterns or watermarks and their positions to compute and account for any rotation, resizing, or other geometric alteration. A system that uses these approaches may not require any frequency transforms or log-polar mappings. Thus, the general idea appears attractive. Also, helper patterns or watermarks can be added to a pre-existing watermark, as long as they do not interfere. In this way, geometric robustness can be added to a pre-existing watermark system.
Unfortunately, such approaches also have drawbacks. For example, there is a tradeoff between robustness and ease of detection of the helper patterns or watermarks. A simple helper pattern may be easy to detect even after it has been geometrically altered—this makes geometric robustness for the main watermark easy to attain, but also makes the helper pattern easy to find and attack. Then, the robustness of the main watermark to geometric manipulations is defeated. On the other hand, the helper patterns may actually be watermarks, which are harder to remove, but also much harder to detect. If the helper watermarks are arbitrary, exhaustive searches may be required over some geometric parameters (such as the angle of rotation). In a sense, such systems merely shift the problem of robustness from the parent watermark to the helper patterns or watermarks.
The present invention overcomes the drawbacks of the foregoing watermark systems by providing a watermark that is resistant to efforts by pirates to avoid “do not copy,” or other messages, of hidden watermarks through techniques such as rotation, resizing or other modification efforts of the protected content.